1. Technical Field
The present invention relates to the plasma processing of semiconductor wafers within a process chamber. More particularly, the present invention relates to an improved electrode for use during the simultaneous plasma processing of multiple semiconductor wafers within a process chamber.
2. Description of the Prior Art
Present practice is to etch semiconductor wafers in a process chamber, often of the type having a bell jar shape. A multifaceted wafer holder is placed within the chamber and a source of RF energy is coupled between the chamber and the wafer holder, which also serves as an electrode, to establish a plasma within the chamber. The use of a multifaceted electrode has the advantage of allowing simultaneous anisotropic plasma etching of several semiconductor wafers, typically 10-24 wafers, depending upon wafer diameter, RF energy levels, etc. See, for example D. Maydan, High Capacity Etching Apparatus and Method, U.S. Pat. No. 4,298,443 (issued Nov. 3, 1981).
During plasma processing in a bell jar shaped process chamber an undesired polymer residue is unavoidably deposited on the inner surface of the bell jar lid. Over time the residue builds up to the point where there is an electrical interaction between the process chamber lid and the electrode during plasma processing. Such residue build-up appears in the form of a distinctive pattern that corresponds to the plan profile of the electrode. As used herein, the electrode is faceted and is described as a pentode or as a hexode, for example, based on the polygonal shape of the electrode outline as defined by the electrode facets, i.e. a pentode is a five-sided or faceted electrode and a hexode is a six-sided or faceted electrode. Thus, in applications where a pentode-shaped electrode is used, a pentagon-shaped pattern is deposited on the inner surface of the process chamber lid; where a hexode-shaped electrode is used, a hexagon-shaped pattern is deposited on the inner surface of the process chamber lid; and so on.
It is thought that such deposits occur because the region between the inner surface of the chamber lid and the electrode is a region of stagnant plasma, i.e. a region of diminished gas circulation and, hence low gas molecule velocity. The chamber is supplied with gas input and evacuation channels and ports, and both are typically relatively remote from this region, thereby contributing to the diminished circulation relative to other areas within the chamber. For example, evacuation ports are typically at the end of the process chamber opposite the lid.
The polymer deposit forms through the dissociation of the various process gases, in combination with the etched metal and carbon, most likely from the photoresist, which forms a carbon matrix or polymer of Al.sub.2 Cl.sub.6, AlCl.sub.3, etc. for etchants containing chlorine, and others for etchants containing fluorine, etc. Such products of dissociation and photoresist breakdown tend to persist in the limited-circulation region of the chamber. In addition, the polymer is preferentially deposited on cooler surfaces within the chamber, such as the region between the inner surface of the chamber lid and the electrode.
The deposited material is hydroscopic, i.e. it tends to fall away from the lid surface in the form of flakes or particles which can come to rest on the surface of the semiconductor wafers undergoing processing within the process chamber, and which can also build up particle residues within the chamber. Therefore, it is necessary to clean such deposits from the process chamber on a regular basis. The present accepted practice is to clean such deposits from the lid of the process chamber after the chamber is deemed unacceptable for further production. The service interval is typically a function of chamber chemistries and operating conditions. If process chamber lid cleaning is postponed, wafer and process chamber contamination becomes a serious problem, adversely affecting process yields.
However, there is a significant downtime penalty associated with cleaning the process chamber lid that adversely affects process throughput. If process chamber lid cleaning could be avoided or at least postponed, the downtime penalty associated with such cleaning could also be avoided or mitigated, resulting in improved process throughput, without negatively impacting process yields.